1. Field of the Invention
The present invention relates to a method of fabricating 3D microstructure, and more particularly, to a method of fabricating 3D microstructure by using a scanning immersion lithography process.
2. Background of the Related Art
3D microstructure plays an important role in the opto-electronic industry. Many fantastic optical functionalities, such as light splitting, focusing, dispersing, polarizing and uniforming, can be realized via engineering the geometry of the optics and their interacting microstructure. Hence, the 3D microstructure is very useful in optical application, such as the light-guide plate of display, the illumination, the optical microlens, the projector and the optical grating.
Traditionally, 3D microstructure is fabricated by high-precision machining. However, this technique can only applied for fabricating 3D microstructure with simple geometry and low precision; It is then not suitable to fabricate novel optics, such as a hybrid lens combining refractive/diffractive functions.
Therefore, using a lithography technique to fabricate the 3D microstructure is a present developing trend, and the lithography technique includes the following methods.
(1) Direct-writing lithography: a laser, e-beam or ion beam is used to provide grey-tone exposure and fabricate the 3D microstructure. This is a simple method, but it is very time-consuming and is not ideal for obtaining reproducible quality.
(2) A grey-tone mask is used to provide grey-tone exposure and then developed to fabricate the 3D microstructure. In this technique, the lithographic process is rather simple, however complex optical simulation, compensation and fabrication processes are required for fabricating the grey-tone mask, in which is very expensive.
(3) Scanning lithography: a relative motion between the mask and the photoresist is performed to provide grey-tone exposure and then developed to fabricate the 3D microstructure. During the scanning and exposure process, an appropriate gap between the mask and the photoresist is required to be maintained for the relative scanning motion. If the gap is too large, the diffraction error is increased to affect the fabrication precision. If the gap is too small, the friction between the mask and the photoresist would destroy the resist surface and hamper the scanning motion.
Synchrotron X-ray was traditionally used as a light source due to its small diffraction error, but the synchrotron source is limited and the vacuum chamber for exposure is not easy to be scaled-up and therefore is unfavorable for fabricating large-area optical device. Ultraviolet (UV) was ever used as the light source to perform the scanning exposure process, wherein the gap between the mask and the photoresist need to be reduced to decrease the diffraction error. However, when the gap is substantially reduced (<50 μm), the thickness uniformity of the photoresist layer and the machine precision are critical, which would markedly increase the cost of scanning stage and therefore unfavorable for fabricating large-area 3D microstructure.
FIG. 1 is a SEM view illustrating a portion of a 3D microstructure fabricated via scanning exposure process with the UV light source, wherein an air gap of 300 μm is sustained between the mask and the photoresist. It is observed that the cross-section of the microstructure 10 is rather irregular and the surface quality is poor, which is not adequate for fabricating high-precision 3D microstructures.